The present invention relates to a method of manufacturing of a passive electro-optic display cell having a helicoidal nematic liquid crystal and two plates, between which is disposed the said crystal. According to this method one effects the deposition of an alignment film on the inner face of each of said plates by evaporation with vacuum under an incidence ensuring an alignment of the molecules of the liquid crystal. The liquid crystal thus becomes orientated along a direction determined by the process of evaporation.
It is to be noted that an alignment film is a thin layer of a material able to orientate the molecules of a liquid crystal when the liquid crystal is placed on said layer. By way of example, such a film can be silicon oxide, SiO.sub.x (1&lt;.times.&lt;2), magnesium fluoride, MgF.sub.2, platinum, chromium, etc. (cf. U.S. Pat. Nos. 3,834,792 and 3,853,391).
The invention has also for an object a passive electro-optic display cell obtained by carrying out this method.
It is known that the substrate of passive electro-optic display cells with nematic liquid crystals must be treated so that an alignment of the molecules of the crystal is realized. Several techniques permitting such an alignment are commonly used.
The first of these techniques consists in rubbing, along a given direction, by means of a rag, the surface of the substrate which will enter into contact with the liquid crystal. The molecules of the liquid crystal which are in the vicinity of the substrate will then be aligned along the direction of the rubbing after the cell is filled. As a matter of fact, once they are orientated, these molecules make an angle of 2.degree. to 3.degree. with respect to the surface of the substrate.
This rubbing technique has the drawback of not being usable in cells the frame of which is made of sintered glass, the properties of alignment of the treated surface formed this way disappearing during the heating of the cell to form the sintered glass. Moreover, the anchoring is relatively weak and an aging occurs which is such that the life of the cell can be jeopardized.
Another technique of orientation of alignment of the molecules of the liquid crystal is by depositing an alignment film, for instance a layer of silicon oxide (SiO.sub.x), under an incidence or angle of the order of 5.degree.. In this case, the projection on the substrate of the direction of alignment of the molecules which are in the vicinity of the substrate coincides with the direction of incidence of the SiO.sub.x, these molecules making then an angle of 15.degree. to 30.degree. with respect to the substrate.
In this case, one ascertains that the contrast of the display as a function of the field applied to the cell varies progressively from a zero field, which can be a drawback for some applications, such as in multiplexed cells.
One can also make the deposition of the layer of SiO.sub.x under an incidence of the order of 30.degree.. In this case molecules of the liquid crystal in the vicinity of the substrate align themselves perpendicular to the direction of the incidence and are, on the average, parallel to the substrate.
In this case, one ascertains that domains (the term being used in the sense it has in crystallography) are formed during the application to the cell of a field higher than a threshold value. These domains are due to the fact that, the molecules being initially parallel to the basis plate, the movement of the molecules can be effected in two opposed directions. Under these conditions, one sees that zones appear which present different optic behaviors, jeopardizing the aesthetic appearance of the cell. It remains that, for some applications, such as in multiplexing for instance, the presence of a threshold is desirable. That would lead then to adopting the solution of the deposition of SiO.sub.x under an incidence of 30.degree., the aesthetic appearance of which is unfortunately not satisfying.
The purpose of the present invention is to remove these drawbacks.